Containers for chemically sensitive materials such as food products traditionally have been made from inorganic materials such as glass. Glass containers are transparent and permit the consumer to view the product before purchasing it. Moreover, glass containers are essentially impermeable to atmospheric gases such as oxygen and hence protect the product. However, glass containers are expensive, heavy and susceptible to breakage. Accordingly, considerable effort has been devoted to development of containers made from polymeric materials such as thermoplastics. Thermoplastic containers can be made inexpensively. They are light in weight and hence inexpensive to ship. They are resistant to breakage and can be fabricated in convenient shapes.
However, polymeric containers ordinarily are permeable to atmospheric gases and to gases in the packaged product. This drawback has limited use of polymeric containers in many applications. Various approaches have been taken towards eliminating the permeability of polymeric containers. Certain polymers have relatively low permeability to particular gases. Containers fabricated from these polymers sometimes can provide satisfactory resistance to permeation for particular applications. However, use of these particular, low permeability polymers can introduce additional problems of cost, transparency, or strength. In certain cases, the low permeability polymers are incompatible with the product to be contained. To alleviate these drawbacks composite containers incorporating one or more layers of a low permeability polymer in conjunction with layers of other polymers have been used. This approach is costly and can make it more difficult to recycle the containers using common recycling techniques such as melt processing.
Various proposals have been advanced for rendering polymeric materials less permeable to oxygen and other gases by depositing thin films incorporating inorganic materials such as oxides of silicon on a substrate consisting of the polymeric material.
Jones, U.S. Pat. No. 3,442,686, notes that pure SiO.sub.2 films or other pure inorganic oxide films deposited by direct vacuum evaporation onto polymeric films form useful oxygen barriers.
White, U.S. Pat. No. 4,667,620, discloses coating of a bottle utilizing direct vaporization and ionization of a metal such as aluminum in an oxidizing atmosphere adjacent the items to be treated. DC and RF biasing is used to accelerate the ions so as to deposit aluminum oxide on the inner surface of the preform or bottle. The reference contemplates deposition of other oxides such as "SiO". White '620 suggests that the coating should be located on the interior of the bottle to better protect the coating and preserve its integrity as an oxygen barrier.
Hahn, U.S. Pat. No. 4,478,874, discloses a generally similar process, except that the same is used to coat the exterior of a bottle. At col. 2, ln. 68-col. 3, ln. 2, the reference notes the possibility of rotating a bottle about its axis to obtain a more even coating.
Felts et al., U.S. Pat. No. 4,888,199, is directed generally to control of plasma processes, but nonetheless discloses a plasma-enhanced chemical vapor deposition process in which a substrate, such as a "metal, glass some plastics and coated substrates" (col. 3, lns. 58-59) is disposed within a plasma of an organosilicon such as hexamethyldisiloxane with oxygen and helium. The reference thus contemplates a direct plasma process, wherein the only plasma in the system is that formed in the immediate vicinity of the substrate. The resulting coating is said to be hard and scratch-resistant.
European Patent Application No. 0,299,754 discloses a direct plasma deposition process generally similar to Felts '199, with specific use of an inert gas, an organosilicon and an oxygen component in the plasma. Among the substrates which can be coated are "various plastics such as polycarbonate resins, useful for packaging foods or beverages", the coating being said to "prevent oxygen or moisture permeation." Example III at pp. 7-8 refers to deposition of coatings having low oxygen permeation and notes that a gas stream incorporating tetramethyldisiloxane or "TMDSO" together with oxygen and helium is useful for that purpose in the direct plasma process.
Plein et al., Plasmapolymerization as Coating Process for Plastic and Metallic Parts (ANTEC, 1988 pp. 1538-1541) describes internal coating of plastic bottles by a direct plasma "polymerization" of hexamethyldisiloxane (HMDSO). HMDSO vapor is introduced through a "monomer inlet" inserted through the mouth of the bottle being coated, the bottle being arranged for rotation during the coating process. The plasma is formed within the bottle itself. This reference states explicitly that the coating formed "does not inhibit the diffusion of oxygen but increases it, depending on the selected substrate", i.e., that the resulting coating is useless as an oxygen diffusion barrier on the bottle.
Despite these and other substantial efforts in the art, thin film coatings incorporating inorganic materials such as oxides have not been widely adopted heretofore in the packaging industry. Each of the processes noted above for making such coatings imposes substantial limitations and drawbacks. Thus, there has been a considerable need for improved processes for coating polymeric articles, and particularly the interiors of polymeric containers with barrier coatings. There have been corresponding needs for improved apparatus for performing the process, and for containers having improved coatings.